Tell us your OS and version of AE down to the last decimal point. Also, is your CUDA up to date? Has this ever worked? Are you using the use unsupported GPU check box? Sorry there's just not enough info in your post for even a suggestion other than checking for updates.
I am using Windows 8.1 pro and After Effects CC Version 126.96.36.199.
My GPU drivers are up to date
I have checked the unsupported GPU box.
It also does not work with CPU only
It's not supported and you have to live with it.
If you want 3d in After Effects, you can use the (much more powerful) Cinema 4D that comes INCLUDED FOR FREE with After Effects CC and later. More info here: details of CINEMA 4D integration with After Effects